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BOEING PROPRIETARY - Copyright (C) - Unpublished Work - See title page for details.
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737-300/400/500MAINTENANCE MANUAL
A 16-bit processor is used as the central processing unit (CPU) for each IRU. Input control signals direct the CPU to perform specific tasks with the aid of software. The program is run at the system clock frequency of 3.84 MHz. The sensor inputs are compensated for inherent drifts, bias, and misalignments. The program uses stored coefficients to perform this function. The inputs are used to compute airplane attitude. This attitude is referenced to the local level navigational coordinates. The angular rate of the airplane over the surface of the earth is also included in the computation. This rate accounts for the airplanes velocity and the earth's rotation. This provides for rotation of the local vertical. Attitude data is also used to resolve acceleration data into the local level coordinate frame. Acceleration is used to compute the airplane's horizontal and vertical velocity. It is also used to compute the airplane's present position.A magnetic deviation look-up table is used to compute magnetic heading. The look-up table contains values of global deviation stored in memory. As the result, heading is computed as a function of present position latitude and longitude. This computed data is added to true heading and true track to produce magnetic referenced data. In the areas beyond 60 degrees South and 82 degrees North, and the magnetic polar regions, the IRS transmits NO COMPUTED DATA (NCD) for magnetic heading. The IRS uses inputs from the ADC to compute windspeed and direction. It also uses these inputs to smooth out the vertical velocity and altitude computations.The IRS uses inputs from the ADC to compute windspeed and direction. It also uses these inputs to smooth out the vertical velocity and altitude computations.When the mode select switch on the MSU is placed in the ALIGN position, the respective IRU enters the alignment mode. The IRU reference axis is aligned to the local vertical. It computes latitude by estimating the horizontal earth rate components. True north is computed from the vector sum of these earth rate components. While in the alignment mode, the IRU will output a signal to light the ALIGN display on the MSU.
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 34-28-01
 ú ú 06 Page 41 ú Jul 15/98

BOEING PROPRIETARY - Copyright (C) - Unpublished Work - See title page for details.
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737-300/400/500MAINTENANCE MANUAL
Minimum of 10 minutes are required to complete the alignment. The system can engage the NAV mode at any time after a satisfactory alignment as determined by the following: 1) The total time in the ALIGN mode is 10 minutes or greater. 2) The quality-of-alignment index is less than 1. (This is a
 measurement of excessive IRU sensor noise or excessive airplane movement during alignment). 3) For latitude: a) The cosine of the entered amount equals the computed cosine amount within 0.01234. b) The sine of the entered amount equals the computed sine amount within 0.15. c) The latitude in memory equals the input latitude within one degree. 4) The longitude in memory equals the input longitude within
 one degree. 5) The Mode Select Switch is set to NAV. 6) There has been no detected motion during alignment.AIRPLANES WITH IRU -109 AND PREVIOUS; If during the alignment process the criteria outlined are not met, several indications occur. If the quality-of-align index is greater than one after ten minutes, the IRU transmits a signal to the MSU. The signal lights the FAULT annunciator and sets the fault ball on the IRU. This fault is also stored in BITE memory.AIRPLANES WITH IRU -110 AND SUBSEQUENT; If during the alignment process the criteria outlined are not met, several indications occur. If the quality-of-align index is greater than one after ten minutes, the IRU transmits a signal to the MSU. The signal lights the FAULT annunciator and is also stored in BITE memory.If the entered latitude fails the sin/cos test after the 10 minute alignment period, the ALIGN annunciator will flash. If the same latitude is entered a second time the flashing ALIGN annunciator will go steady and the FAULT annunciator will light. If the second latitude entry does not agree with the first entry, the ALIGN annunciator will continue to flash. A FAULT is annunciated only after the same latitude is entered a second time (after a 10 minute alignment period) and fails the sin/cos test after the second entry. However, a latitude may be entered at anytime during the alignment process. If it passes the tests, the system will enter the NAV mode.
　
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